1. Field of the Invention
The present invention relates to an apparatus and a method for balancing SOC (State Of Charge) of a plurality of cells in a battery, and in particular, to an apparatus and a method for balancing SOC of cells in a battery using a voltage variation pattern of each cell.
2. Description of the Related Art
Generally, a battery used in a high output product such as an electric vehicle should supply a high voltage to a load, and thus contains a plurality of cells connected in series or parallel.
When the battery containing a plurality of cells gets discharged, SOC of each cell becomes different as the usage time of the battery is increased due to the difference in self-discharge rate between the cells. If the battery continues to get discharged in spite of the unbalanced SOC, a specific cell having a lower SOC is over-discharged, resulting in unstable operation of the battery. To solve the unbalanced SOC problem, conventionally a boost or buck circuit is installed at each cell. The boost or buck circuit charges a cell having a relatively lower SOC or discharges a cell having a relatively higher SOC to balance SOC of the cells.
To balance SOC of cells, it is necessary to accurately measure SOC of each cell. However, it is impossible to directly measure SOC of a cell due to the non-linearity characteristics of battery behavior. For this reason, SOC of a cell is indirectly estimated using electrical parameters that can be measured, such as OCV (Open Circuit Voltage), a discharge current and so on.
A method for estimating SOC of a cell using OCV determines OCV of a cell using an output voltage of the cell, and maps the OCV to a corresponding SOC using a lookup table including SOC data based on OCV.
However, a measured output voltage of a cell is different from an actual output voltage of the cell due to IR drop that may occur when a battery is charged or discharged. As a result, the method for estimating SOC of a cell by considering an output voltage of the cell as OCV has a disadvantage of low accuracy.
For reference, IR drop means a phenomenon that voltage changes rapidly when a battery starts to get charged or starts to get discharged while connected to a load. That is, a cell voltage drops rapidly when discharge starts, and a cell voltage rises rapidly when charge starts.
A method for estimating SOC of a cell by integrating the charge/discharge current estimates SOC of a cell using an integrated amount of charge/discharge current of the cell. This method has an advantage of simple estimating. However, current measurement errors are continuously accumulated over time, resulting in low accuracy.
Recently, various methods for estimating SOC of a cell using mathematical models have been suggested. According to the method, SOC of a cell is estimated using various electrical parameters that can be measured, such as voltage, current, temperature and so on. However, because the mathematical model is complex, it takes a long time to estimate SOC of a cell, and it requires a high-grade computing hardware for a complex calculation.